For portable electronic devices such as mobile phones and notebook computers, a lithium ion secondary battery comprising a positive electrode, a negative electrode and a non-aqueous electrolyte solution for secondary batteries has been widely used. As a solvent of a non-aqueous electrolyte solution for lithium ion secondary batteries, a carbonate type solvent (such as ethylene carbonate or dimethyl carbonate) has been widely used in that it dissolves a lithium salt excellently to provide a high lithium ion conductivity, and it has a wide potential window. However, since a carbonate type solvent is flammable, there is a risk of ignition e.g. by heat generation of the battery.
Thus, as a non-aqueous electrolyte solution excellent in flame retardance, a non-aqueous electrolyte solution using a fluorine-containing solvent has been proposed.
For example, as a flame retardant non-aqueous electrolyte solution having favorable battery properties (the cycle properties and the discharge capacity), a non-aqueous electrolyte solution comprising a fluorine-containing solvent, a non-fluorinated cyclic carbonate, a non-fluorinated cyclic ester and a lithium salt has been known (Patent Document 1).
However, according to studies by the present inventors, if a lithium ion secondary battery using the non-aqueous electrolyte solution in Patent Document 1 is stored at high temperature, its discharge capacity is likely to decrease, it is likely to generate a gas, and its high temperature storage properties are insufficient.
As a method for increasing high temperature storage properties of a non-aqueous electrolyte solution using a carbonate type solvent, it has been proposed to incorporate at least one of a monofluorophosphate and a difluorophosphate (Patent Document 2).
However, incorporation of a monofluorophosphate or a difluorophosphate into a non-aqueous electrolyte solution as in Patent Document 1 has not been studied, and a non-aqueous electrolyte solution having both flame retardance and high temperature storage properties has not been obtained yet.
In general, during use of a secondary battery, the battery temperature rises due to e.g. Joule heat or heating from outside, and if the battery temperature reaches a high temperature at a level exceeding 150° C., thermal runaway may occur to cause breakage of the battery. Such thermal runaway is known to be caused by heat generation which occurs when components in the electrolyte solution are reacted with electrodes and decomposed. Therefore, it is important for a lithium ion secondary battery that the non-aqueous electrolyte solution has a low reactivity with electrodes and is less likely to cause heat generation.